In a press section of early paper and board machines, a roll was used which was commonly made of granite. In spite of the excellent surface properties of granite rolls, rolls having a ceramic coating have been gradually substituted for granite rolls. In prior art rolls with metal, metal-ceramic and ceramic coatings, however, corrosion of various degrees has occurred in particular in more severe conditions, which corrosion causes corroding of the roll, delamination of the coating, and wear of the roll so that it becomes unusable.
These types of rolls are coated by means of thermal spraying, in which case the coating unavoidably remains porous, and the roll face susceptible to corrosion. In view of corrosion, the process conditions in paper industry have become more and more severe because, for reasons of protecting the environment, process water is recirculated to an ever greater extent, and the rolls also must operate under ever more corroding conditions. For this reason, it is necessary to renew the coatings on rolls and/or to replace the rolls quite frequently which, of course, causes high expenses.
Attempts have been made to improve the resistance of rolls to corrosion by means of a variety of different methods.
In Finnish Patent Application No. 853544, a method for coating a roll is described in which a metallic core of a roll has been coated fully or partly with a metal, such as stainless steel, and in which the proportion of chromium is from about 9% to about 35%. It has been believed that the presence of chromium in the steel expressly increases the resistance of steel to corrosion.
In Finnish Patent No. 89,950, the metal core of a roll has been coated with molybdenum- and nickel-based metals, such as nickel-chromium alloys, nickel-chromium aluminum alloys, etc., in order to improve the resistance of the roll to corrosion. This intermediate layer is applied onto the roll by means of flame spraying or plasma spraying. The thickness of the intermediate layer is from about 100 .mu.m to about 500 .mu.m. Between this porous metal layer and the core of the roll, it is further possible, by means of flame spraying or plasma spraying, to apply a tight metal film that inhibits corrosion and that has a thickness of about from about 100 .mu.m to about 500 .mu.m. Metals suitable for this are nickel, aluminum alloy, copper, stainless steel, etc., and the particles that are used for the spray coating material are very small.
In Finnish Patent No. 86,566, a roll coating is described including a corrosion-protection layer on the roll core and which has a thickness of at least about 0.5 mm. The tightness of the corrosion-protection layer is higher than about 96% and, if necessary, the corrosion-protection layer may have been sealed by means of laser, induction, plasma, flame, or electron-jet melting, or it may also have been produced by means of thermal spraying. The corrosion-protection layer consists of stainless steel whose chromium content is from about 10% to about 29%, and it is placed between the core mantle of the roll and the ceramic outer layer.
In Finnish Patent No. 82,094, a coating for a Yankee cylinder in a paper machine is described, whose resistance to corrosion permits manufacture of special papers in acid solutions, in which the pH can be in the range from about 3 to about 5. The coating has been made of a mixture of a metal powder and a carbide or nitride.
In Finnish Patent No. 84,506, a press roll for a paper machine is described in which the roll core has been coated with an intermediate layer which consists of a composite compound made of a ceramic and a metal, so that the ratio of the components in the composite compound is different in different parts of the intermediate layer in the direction of the radius of the roll. The intermediate layer is then coated with a ceramic material over the intermediate layer.
In European Patent Application No. 0 657 237, coating of a roll with tungsten carbide and chromium carbide by means of thermal spraying is described. The function of this coating is also, among other things, to improve the resistance to wear of the carbide coatings on rolls.
In European Patent Application No. 0 481 321, a press roll is described in which an intermediate layer made of a molybdenum-based or nickel-based alloy has been applied onto the metal core, for example, by means of plasma spraying. A ceramic layer is applied onto this layer by means of the plasma spraying method, which ceramic layer consists of metal oxides or mixtures of same. The roll is then coated with an organic polymer to fill the pores in the porous ceramic coating.
In the prior art methods and in the coatings that are used currently on the rolls in a paper machine, particularly the center roll of a press section of a paper machine, a problem is the softness of the polymer coating or the porosity of the ceramic coating. As a result, the medium that produces corrosion of the roll frame, such as moisture and other corroding materials, can penetrate through the small pores through the coating to the roll frame. In this connection, corrosion arises, which deteriorates, among other things, the adhesion of a ceramic coating to the roll and promotes damage to the metal core of the roll. In particularly difficult corrosive environments, such as rolls that operate under highly acid or alkaline conditions, very rapid corroding has been noticed in spite of a metallic corrosion-protection layer that has been applied underneath a ceramic coating by means of plasma spraying or high-velocity flame spraying (HVOF) and whose thickness is from about 200 .mu.m to about 300 .mu.m. In such a case, the coating layer of the roll often falls apart partially, i.e., delamination takes place, and the roll is unusable.